dt-bindings: net: altr,socfpga-stmmac.yaml: add minItems to iommus

dt-bindings: net: altr,socfpga-stmmac.yaml: add minItems to iommus

Add missing 'minItems: 1' to iommus property of the Altera SOCFPGA SoC
implementation of the Synopsys DWMAC.

Fixes: 6d359cf464f4 ("dt-bindings: net: Convert socfpga-dwmac bindings to yaml")
Signed-off-by: Matthew Gerlach <matthew.gerlach@altera.com>
Reviewed-by: Yanteng Si <siyanteng@cqsoftware.com.cn>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20250707154409.15527-1-matthew.gerlach@altera.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>